Switching power supply controller with built-in supply switching

ABSTRACT

The invention provides a switching converter comprising as few as two high-side switching transistors and a low-side rectifying device, along with a control circuit. The switching converter is capable of operating from a main supply source or an auxiliary supply source. The invention further includes a method for producing a regulated voltage from a first supply voltage and a second supply voltage via the two high-side switching transistors and a low-side rectifying device.

CROSS-REFERENCES

This application claims the benefit under 35 U.S.C. § 119(e) ofprovisional application No. 60/545,339 filed Feb. 17, 2004, which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a switching power supply circuit forelectronic devices.

BACKGROUND OF THE INVENTION

Modern computers are generally designed to receive expansion cards thatadd functionality to the computer. Such expansion cards may include, forexample, a LAN network interface card, a wireless LAN card, a graphicaccelerator card, etc., and are typically designed to be compatible witha given industry specification (e.g. mPCI, Cardbus, PC-card, etc.).These expansion cards are typically plugged in to the “host” computerand operate from the host's power supply or supplies. Certain industryspecifications (e.g., the mPCI specification) presently requireexpansion cards to operate either from a main power supply or anauxiliary power supply (e.g., derived from a battery), which typicallyprovide supply voltages of either 3.3V or 5.0V to the various circuitsin the computer. In general, however, due to advances in integrationtechnology and power management, modern integrated circuits (“ICs”)typically are designed to operate from a supply voltage of 3.3V (ratherthan 5.0V), and many are now designed to operate from a supply voltageof 1.5 V.

For these reasons, expansion cards conventionally include a powercontroller to select either a main or auxiliary supply voltage (whichmay be either 3.3V or 5.0V) from a host computer and convert theselected supply voltage to the voltages that are needed by the IC's onthe expansion card. The power controller conventionally also functionsas an on/off switch for the expansion card, so that the CPU in the hostcomputer may shut down the expansion card as needed, e.g., to save powerin a standby mode. It further conventionally includes a “bypass” circuitthat is used to pass one of the supply voltages directly to the ICs onthe expansion card without any voltage conversion, e.g., when the hostvoltage is so close to the voltages needed by the expansion card thatvoltage conversion is impossible. The power controller may also includecircuits for monitoring the host main and auxiliary supply voltages andfor sending a “reset” or shut-down signal to the ICs on the expansioncard in the event of an overvoltage or undervoltage condition or inresponse to a RESET command from the host computer. Finally, the powercontroller may also include a standby supply circuit that provides powerto certain circuits on the expansion card that remain active even whenthe expansion card is placed in standby mode (e.g., a wake-up circuit).

These features have conventionally been implemented via acustom-designed power controller circuit using a large number ofdiscrete components and ICs. For example, a conventional powercontroller may require more than 28 discrete components, including aswitching IC for on/off switching, a supply selection switch IC, one ormore “main” DC/DC converter ICs having a linear regulator or ahigh-efficiency switched mode power supply (“SMPS”) converter, a“standby” supply DC/DC converter IC, and several supply monitoring andreset logic circuits including internal references, voltage comparators,time-delay circuits, etc.

FIG. 1 illustrates the manner in which supply selection and voltageconversion have been implemented in conventional expansion card powercontrollers. The host main and auxiliary supply voltages are received atterminals 102 and 100, respectively, and are connected to supplyselection switch IC 108 (an SPDT-type switch) via terminals 104, 106.The selected output voltage at node 110 is then input to one or moreDC/DC converter ICs 118. As shown in FIG. 1, the DC/DC converter ICs 118are conventionally either switching-type converters (including two FETswitches 114, 116, a pass inductor L1, and a shunt capacitor C1, asshown) or linear-drop-out regulators.

FIG. 2 provides a more detailed illustration of the conventional powercontroller circuit shown in FIG. 1. Supply selection switch IC 108 isconventionally an IC having two high-power, low-impedance FETs Q1 and Q2and associated switching control circuitry. Switching transistors Q1 andQ2 are connected to the main supply voltage via IC pin 206 and theauxiliary supply voltage via IC pin 208, and their source terminals areconnected together (at node 210) to IC pin 212.

DC/DC converter IC 114, as shown in FIG. 2, includes transistors Q3 andQ4, which operate essentially as switches that are either open orclosed. Transistors Q3 and Q4 are controlled via control logic 220. Thesource terminal of transistor Q3 and the drain terminal of transistor Q4are connected via IC pin 222 to series inductor L1. Inductor L1 in turnis connected to the output node 234, where the regulated voltage isoutput to the other circuits on the expansion card. Capacitor C1 isconnected from node 236 to ground, in order to stabilize the outputvoltage against transients that the supply selection switch 108 and andany bypass circuitry (not shown) tend to create. The output voltage istaken at node 234 and also fed back via IC pin 224 to control logic 220.

As is known in the art, DC/DC converter IC 114 operates by switching thehigh-side power transistor Q3 in a pulse-width-modulated manner, whilesimultaneously switching the low-side transistor Q4 in an oppositefashion. In other words, when transistor Q3 is open, transistor Q4 isclosed, and vice versa. As such, the source voltage at pin 216 isperiodically connected to inductor L1 and capacitor C1. The voltagedeveloped across capacitor C1 powers the load at node 234. In addition,the output voltage is typically sensed, such as by a voltage divider,and fed as one input to an error amplifier (in control logic 220). Areference voltage is fed to a second input of the error amplifier. Theoutput of the error amplifier feeds one input of a comparator (also incontrol logic 220). The other comparator input is typically fed by aperiodic control waveform, such as a triangle wave. The comparator, inturn, operates the power switch with a series of control pulses, thewidth of which are used to regulate the load voltage to the desiredlevel despite fluctuations in the load.

In conventional expansion cards, additional power converters or linearregulator ICs (LDO1 and LDO2, not shown) may further be connected to ICpin 212 of the supply selection switch 108. These additional regulatorsmay be used to provide additional supply voltages that may be needed bythe circuits on the expansion card (e.g., 1.5 V).

It will be recognized that the conventional power controller describedabove is both complex and expensive. The power controller for eachexpansion card is conventionally custom designed. Although customdesigns provide the benefit that the power controller can be optimizedfor a given expansion card's power requirements, the labor cost requiredto design a conventional power controller is very high. Because of thishigh labor cost and the cost of the numerous discrete componentscontained in the conventional power controller, the conventional powercontroller represents a substantial part of the overall cost of anexpansion card. It would therefore be desirable to provide a powercontroller that could be integrated onto a single monolithic integratedcircuit with a reduced number of components.

SUMMARY OF THE INVENTION

The present invention provides a monolithic, highly integrated powersupply controller circuit capable of providing various voltages forcircuits on an expansion card, either from a main supply source or anauxiliary supply source.

The invention provides a dual-supply switching converter comprising asfew as two high-side switching transistors and a low-side rectifyingdevice. In accordance with the invention, the two high-side switchingtransistors each are connected to a different power supply. Operationfrom either power supply is then made possible by disabling thehigh-side transistor connected to the non-selected power supply, andthen operating the high-side transistor connected to the selected powersupply in conjunction with the low-side rectifying device to produce aswitched, regulated output in the conventional manner. Transfer from onepower supply to the other is accomplished via a break-before-maketechnique (i.e., the operating high-side transistor is disabled beforethe other high-side transistor is caused to begin operating as ahigh-frequency switch).

The invention further provides a method of producing a regulated voltagefrom a first supply voltage and a second supply voltage, comprising thesteps of: (a) providing the first supply voltage to the first high-sideswitch; (b) providing the second supply voltage to the second high-sideswitch; (c) selecting one of the first and second high-side switches tobe a first active switch and the other one of the first and secondhigh-side switches to be a first inactive switch; (d) deactivating thehigh-side switch selected as the first inactive switch; (e) switchingthe high-side switch selected as the first active switch at a switchingfrequency to produce a switched output signal; and (f) rectifying theswitched output signal to produce a regulated output signal.

The invention still further provides a multiple-output power supplydevice, comprising: a monolithic integrated circuit including a firstswitching converter on a first portion of the monolithic integratedcircuit; a second switching converter on a second portion of themonolithic integrated circuit; and a control circuit connected to thefirst and second switching converters and capable of enabling anddisabling one or both of the first and second switching converters. Thefirst switching converter may include a terminal for receiving a firstsupply voltage and a terminal for receiving a second supply voltage. Thefirst switching converter may further be capable of converting aselected one of the first and second supply voltages to a regulatedoutput signal.

The invention also provides a method for providing a plurality ofregulated output voltages via a single monolithic integrated circuit,comprising the steps of: providing a first switching converter on afirst portion of the monolithic integrated circuit; providing a secondswitching converter on a second portion of the monolithic integratedcircuit; providing a control circuit connected to the first and secondswitching converters; and enabling and disabling one or both of thefirst and second switching converters via the control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described indetail in conjunction with the annexed drawings, in which:

FIG. 1 is a block diagram depicting a conventional power controller foran expansion card;

FIG. 2 is a block diagram further depicting the conventional powercontroller of FIG. 1;

FIG. 3 is a block diagram depicting a power controller in accordancewith the invention;

FIG. 4 is a block diagram further depicting a power controller inaccordance with the invention;

FIG. 5 is a timing diagram illustrating the operation of a powercontroller in accordance with the invention; and

FIG. 6 is a block diagram depicting a further embodiment of a powercontroller in accordance with the invention.

DETAILED DESCRIPTION

As described above, the present invention is a highly integrated powercontroller for providing various voltages to circuits on an expansioncard. The extensive level of circuit integration in the presentinvention is accomplished by a unique combination of the functionsconventionally performed by a voltage supply selection IC with thefunctions conventionally performed by a switching DC/DC converter.

FIG. 3 is a block diagram depicting a dual-supply switching converter(“DSSC”) in accordance with this aspect of the invention. As in FIG. 1,the main and auxiliary supply voltages are input at terminals 302 and300, and respectively provided to terminals 304 and 306 of the supplyselection switch 308. In accordance with the invention, the two SPSTswitches that make up the supply selection switch 308 (an SPDT switch)are operated such that the connection between terminals 304 and 310 (forauxiliary supply) or between terminal 306 and 310 (for main supply) isswitched at a high frequency, in a similar manner as high-side switch114 was switched in the conventional DC/DC converter 118. Low-sideswitch 312 is likewise switched at high frequency (albeit in reverse ofthe switches in supply selection switch 308) as in the conventionalDC/DC converter 118. The result is that a pulse-width-modulated currentpasses through inductor L1 to charge capacitor C1, thus supplying theloads connected via terminal 314 with a regulated voltage.

FIG. 4 further depicts the dual-supply switching converter (“DSSC”) ofFIG. 3. In FIG. 4, the SPDT supply switch is formed by high-sideswitching transistors Q1 and Q2, and the low-side switch 114 is shown astransistor Q3. Control circuit 406 monitors the various voltages andcurrents in the DSSC and controls the switching of transistors Q1, Q2and Q3 in FIG. 4. Inductor L1 and capacitor C1 are connected to outputpin 416 of the DSSC. Assuming that a high switching frequency is used(e.g., 1 MHz), inductor L1 and capacitor C1 may be relatively small(e.g., about 2.2 to about 4.7 uH, and about 10 uF to about 22 uF,respectively) and may be implemented via discrete components connectedto the DSSC IC 400. Capacitor C1 may be a low-cost, nonexplosive ceramictype capacitor.

Control circuit 406 may be implemented via combinational logic, as in anASIC, or via a microcontroller or simple microprocessor. Control circuit406 preferably includes an algorithm for determining whether the mainsupply voltage or the auxiliary supply voltage should be selected as thesupply source at a given time. This algorithm is predetermined and maybe based on a variety of factors, including the quality or voltage levelof the main and auxiliary supply voltages Vcc1 and Vcc2 or the receiptof a predetermined control signal from a host computer.

After control circuit 406 determines that either the main supply voltageor the auxiliary supply voltage should be selected as the supply source,control circuit 406 outputs a suitable control signal to the gate of thehigh-side transistor associated with the non-selected supply source(e.g., Q1), so that that transistor is placed into a fully nonconductivestate. Control circuit 406 further produces control signals causing thehigh-side transistor associated with the selected supply (e.g., Q2) toopen and close at a predetermined rate and with a predetermined pulsewidth, and causing the low-side transistor Q3 to operate in reverse ofthe selected high-side transistor, as in a standard single-sourceswitching converter. The above control signals may be produced inaccordance with techniques well-known to those skilled in the art ofswitching DC/DC converters. In this manner, dual-supply switchingconverter 400 is capable of selecting one of the supply source voltages(main or auxiliary) and converts the selected voltage to a lower voltageat output terminal 424.

DSSC 400 may further include current sensing devices 410 and 412 to feedback a small portion of the current flowing through high-sidetransistors Q1 and Q2 to the control circuit 406. These current sensingdevices 410 and 412 may be implemented via small transistors connectedin parallel with high-side transistors Q1 and Q2, in the manner known tothose skilled in the art. Control circuit 406 may then also monitor thecurrent through the selected transistor (Q1 or Q2) and stabilize itusing conventional cycle-by-cycle current limiting techniques.

In an alternative embodiment, low-side transistor Q3 may be replaced bya high-power Schottky rectifying diode of the type conventionally usedin certain conventional single-transistor switching DC/DC convertercircuits.

DSSC 400 may also include a tub control circuit 408 to control thebiasing of the n-tub (also known as the n-well) of switching transistorsQ1 and Q2 during switching and transfer from one source to the othersource. Tub control circuit 408 monitors the output voltage, the twoinput voltages, and the voltages at various locations in the n-tub andproduces a tub terminal bias voltage that is sufficient to reduceparasitic currents in the parasitic elements inherently present intransistors Q1 and Q2. The tub control circuit may be implemented viaknown techniques, such as those used in charge pump circuits. Inaddition, because the tub current may be as high as several hundredmilliamperes, the n-tubs for each of transistors Q1 and Q2 should beconnected via large tub ties so that the voltage drop across the tubties is minimized.

FIG. 5 depicts control timing suitable for operating the DSSC. In FIG.5, traces 500 and 502 depict the voltage at the main power supply (PS1in FIG. 5) and the voltage at the auxiliary power supply (PS2 in FIG.5), respectively. Trace 504 depicts an internal signal STOP_SW withincontrol circuit 406 that is used to temporarily halt the cycling of theDSSC during the switch-over from one supply source to the other. Traces506 and 508 depict two other signals SEL_PS1 and SEL_PS2, also internalto control circuit 406, that indicate whether the main power supply PS1or the auxiliary power supply PS2 is to be used as the selected powersource. Finally, trace 510 labeled LX OUTPUT depicts the DSSC outputsignal at pin 416, which is passed to the series inductor L1.

Initially, both the main power supply voltage and the auxiliary powersupply voltage are shown as “off.” At time 512, the main power supplyvoltage ramps up to its normal level (e.g., 5.0 V), and the DSSCcommences switching Q1 and Q3 to generate the LX square-wave output. Attime 514, the auxiliary power supply ramps up to its normal level. Attime 516, the control circuit determines that the DSSC should switchfrom the main power supply to the auxiliary power supply. Thisdetermination may be made in accordance with a predetermined algorithmbased on a variety of factors, including the quality or level of themain and auxiliary supply voltages and the “preferred” source for agiven application, such as utility power rather than battery power.

The determination to switch from one supply to the other is reflected inFIG. 5 by the STOP_SW signal going “high.” This STOP_SW signal causescontrol circuit 406 to cease switching either of the high-sidetransistors while the transfer is made from one supply to the other(i.e., during time period dt3). The cessation of switching is reflectedon LX OUTPUT at time 522, in that the LX OUTPUT stays “low” whileSTOP_SW is “high.” After the STOP_SW signal goes “high,” and furtherafter a short time delay DT1, the SEL_PS1 signal goes “low,” thuscausing control circuit 406 to cease switching high-side transistor Q1and instead to place it into a nonconductive state, and further to“open” transistor Q3 so that current is enabled to continue flowing inthe loop formed by transistor Q3, L1, the load circuits and ground.After a further short time delay DT2, at time 520 the SEL_PS2 goes high,thus indicating that control circuit 406 should commence switchingtransistor Q2 associated with the auxiliary power source. Accordingly,at time 524 the STOP_SW signal is released and the control circuitcommences switching transistors Q2 and Q3 to produce LX OUTPUT signalsonce again, from the auxiliary power source. It may be seen from theabove sequence that the transfer from transistors Q1 and Q2 is made in abreak-before-make manner.

Between times 524 and 530, the DSSC continues running from the auxiliarypower supply. If, for some reason, the auxiliary power supply signal PS2should turn off or become invalide (as shown at time 528), the DSSC willtransfer back to the main power supply. Accordingly, at time 530 theDSSC is turned “off,” high-side transistor Q2 is deselected (after delayDT4) and high-side transistor Q1 is reselected (after delay DT5). Aftera total delay DT6, at time 540 the DSSC is turned back on and the LXoutput is restarted.

Preferably, the transfer from one supply source to the other is made asrapidly as possible, in order to avoid transient voltage effects fromappearing at output Vout. For example, if the DSSC is operated at a 1.44MHz switching frequency (or about a 700 ns clock cycle), the transfer ispreferably accomplished within one clock cycle.

In a further embodiment, an optional inductor bypass transistor Q4(shown in phantom in FIG. 4), may be provided across inductor L1, andthe inductor current monitored during the period of time in whichSTOP_SW is “high.” If the inductor current begins to approach zeroduring time periods dt3 or dt6, then the inductor bypass transistor Q4may be turned “on,” and low-side transistor Q3 turned “off.” In thismanner, reverse current that might tend to flow from capacitor C1through the inductor and through transistor Q3 to ground may be avoided,and the output voltage Vout will simply be maintained at approximatelyits pre-transfer level by capacitor C1.

The DSSC as described above represents a significant improvement overconventional power controllers. Conventional power controllers requiretwo high-power switching transistors in a voltage supply selection ICand two additional high-power switching transistors in a switching DC/DCconverter IC. Thus, conventional power controllers require a total offour transistors, each of which must be a relatively large, high-power,low-impedance device. In contrast, the present invention requires onlythree transistors—two high-side transistors for selecting one of the twovoltage supply sources and for providing high-side switching, and onelow-side transistor for maintaining the flow of current through theseries inductor and the load when the high-side transistors are “off.”Alternatively, if a rectifying diode is used as the low-side rectifyingdevice, then the present invention only requires the two high-sidetransistors and the diode. Moreover, the separate ICs in conventionalpower controllers require separate IC packaging, separate pins, separatepower leads, separate control circuits, separate I/O circuits, etc.Using the DSSC in the present invention, these redundant elements areeliminated. The overall reduction in the number of circuit elements hasyielded a DSSC that is significantly smaller than conventional powercontrollers.

Based on the reduction in circuit size attributable to the DSSC, thepresent inventors have succeeded in creating a multi-function powercontroller on a single integrated circuit having a die area of only 4.0mm². FIG. 6 depicts a multi-function power controller (“MFPC”) accordingto this aspect of the invention. As shown in FIG. 6, MFPC 600 maycomprise two dual-supply switching converters (“DSSCs”) 623,635 thatoperate as described above with reference to FIGS. 3–5. Each DSSC623,635 preferably receives both a main power input (VCC_3.3, VCC_2.0)and an auxiliary power input (AUX_3.3, AUX_2.0). These power inputspreferably have voltages that are between 2.5V and 5.5V, and morepreferably that are 3.3V or 5.0 V, e.g. in compliance with current(m)PCI specifications. Each DSSC 623,635 comprises three transistors(Q1–Q3; Q4–Q6) that are controlled by control circuits 622 and 636. Tubcontrol for transistors Q1, Q2, Q5 and Q6, as described above withreference to FIGS. 4 and 5, is also provided by control circuits 622 and636 via connections 614 and 634. Finally, each DSSC 623, 635 has aswitched output (LX3P3, LX_2.0) for connection to series inductor L1, L2and shunt capacitor C1, C2, respectively. The regulated supply voltageoutput from each DSSC is shown at nodes 606 and 612.

In order to provide a variety of regulated supply voltages typicallyneeded by various integrated circuits in a PC expansion card, e.g., awireless network interface card, the first DSSC 623 (comprising Q1–Q3)preferably converts the selected main or auxiliary supply voltage to aregulated “main” 3.3V power at node 606, while the second DSSC 635(comprising Q4–Q6) preferably converts the main or auxiliary power inputto a lower voltage, such as 2.0 V, at node 612. Each DSSC 623,635further includes a feedback connection between nodes 606, 612 andterminals FB_3.3 and FB_2.0, which are further connected (within DSSCcontrollers 622, 636) to resistive voltage dividers 618, 620 and638,640. The feedback voltage output from the voltage dividers is thenused to adjust the frequency of the switching of the switchingtransistors Q1–Q6. The oscillator and ramp generator signals customarilyneeded by switching converters are generated in block 628.

The MFPC 600 further may comprise two low-drop-out regulators (“LDO1”and “LDO2”) 646, 648. As shown in FIG. 6, LDO1 and LDO2 preferablyreceive a 2.0V input, taken from the regulated output 612 of the DSSC635. LDO1 and LDO2 efficiently convert the 2.0V input down to a 1.5Vregulated output 650, 654. Capacitors C6 and C7 are connected betweenregulator outputs 650, 654 and ground, respectively, and serve tostabilize the output voltage at outputs 650, 654. Advantageously, thetwo outputs from LDO1 and LDO2 may be used to supply expansion cardcircuits that otherwise might interfere with one another if suppliedfrom a single voltage source. For example, if the MFPC 600 is applied ina wireless LAN card, the output of LDO1 may be used to supply the analogcircuits of the physical interface (PHY) in the LAN card, while theoutput of LDO2 may be used to supply the core digital circuits of thePHY interface.

The MFPC 600 further may include a stand-by supply, e.g., for supplyinga WMAC standby current and supplying power to the various control andlogic circuits on the MFPC 600. The stand-by supply is preferablyprovided via a separate host supply source-selection switch in block 624(connected to VCC_2.0 and AUX_2.0), followed by a third low-drop-outregulator 642 that preferably has a very low quiescent current.Regulator 642 preferably provides a 3.3V supply voltage needed by thelogic circuits in the two DSSC controllers 622, 636. In order to savepower in standby mode, regulator 642 is preferably a low-quiescentcurrent device having a quiescent current of no more than 10 mA.

The MFPC 600 may also provide a bypass feature. As described in thebackground section above, a bypass feature is conventionally used topass a supply source voltage from the input of a DC/DC converterdirectly to the output of the DC/DC converter, via a separate, discretebypass transistor IC. In the present invention, however, this functionis accomplished by placing the shunt transistors (Q3 or Q6) into anonconductive state (i.e., turned “off”) while either the main orauxiliary switching transistors (Q1,Q2 or Q4,Q5, depending on thedesired supply source, either main or auxiliary) are placed into a fullyconductive state. Thus, the switching transistors are essentially placedinto a 100% duty cycle. As a result, the output of the DSSCs 623,635will be the voltage of the selected input (main or auxiliary) minus therelatively small resistive voltage drop of the switching transistorsthemselves. In this manner, the present invention provides a bypassfunction without requiring a separate bypass transistor IC.

MFPC 600 further may include a reset circuit, shown in block 628. Thereset circuit includes overvoltage and undervoltage comparators thatmonitor the voltages received and/or generated by the various DSSCs andregulators on the MFPC 600. If they are out of range, the reset circuitgenerates a RESET_N signal, which indicates to other circuits connectedto the MFPC 600 that the MFPC voltages are out-of-range. This supplyvoltage monitoring functionality removes the need for external resetcircuitry.

The reset circuit further includes a separate reset-input pin “PHYRES”that allows for external reset events, e.g., from a host computer. Thereset circuit will activate the RESET_N signal if it receives the PHYRESsignal. The reset circuit will also activate the RESET_N signal duringthe initialization and power-up stage of MFPC 600.

The MFPC 600 may also include thermal monitoring and shutdown circuits(block 630). If the temperature of the MFPC IC rises above a temperaturethat would cause irreversible damage, the thermal monitoring andshutdown circuits disable MFPC 600 and thereby prevent from causingdamage to itself or to other circuits on the expansion card. Block 630may further include circuits for generating one or more referencevoltages used in the MFPC 600.

Advantageously, the various DSSCs and regulators on the MFPC may beindividually controlled (i.e., turned on and off) by control circuit626. In the embodiment shown in FIG. 6, the MFPC receives three externalcontrol signals PSW1, PSW2, and PSW3. As an example, a signal on PSW1may control whether the 2.0V DSSC 635 and the two 1.5V LDOs 646, 648 areactive or shut down, while a signal on PSW2 may control whether the 3.3VDSSC 623 is active or shut down. Finally, a signal on PSW3 may controlwhether the 3.3V DSSC 623 is to be placed into bypass mode or not (i.e.,should pass Vcc or Vaux directly to the LX_3.3 output, as describedabove). In this manner, the MFPC may receive commands from a hostcomputer (e.g., to place an expansion card into various active, “sleep,”and “deep sleep” modes) and activate or deactivate the various PSSCs andregulators on the MFPC 600 in response to those commands.

It will be recognized that MFPC 600 is not limited solely to aparticular host computer configuration, or solely to applicationsinvolving expansion cards. Rather, it may be utilized in any circuitrequiring the various voltages that the MFPC 600 is capable ofsupplying.

While the invention has been described with reference to the preferredembodiment thereof, it will be appreciated by those of ordinary skill inthe art that modifications can be made to the structure and elements ofthe invention without departing from the spirit and scope of theinvention as a whole.

1. A switching converter capable of operating from a first supplyvoltage and a second supply voltage, comprising: a first high-sideswitch having a control terminal, an output terminal, and an inputterminal capable of receiving the first supply voltage; a secondhigh-side switch having a control terminal, an output terminal, and aninput terminal capable of receiving the second supply voltage, whereinthe output terminal of the second switch is connected to the outputterminal of the first switch; a low-side rectifier, connected betweenground and the node formed by the output terminals of the first andsecond high-side switches; and a control circuit connected to thecontrol terminals of the first and second high-side switches; whereinthe control circuit causes a selected one of the first and secondhigh-side switches to switch repetitively between a conductive state anda nonconductive state at a predetermined switching frequency and causesthe nonselected one of the first and second high-side switches to remainin a nonconductive state.
 2. The converter of claim 1, furthercomprising: a capacitor having a grounded terminal and a nongroundedterminal, and wherein the nongrounded terminal of the capacitor isconnected to the node formed by the output terminals of the first andsecond high-side switches.
 3. The converter of claim 2, furthercomprising: an inductor connected between the ungrounded terminal of thecapacitor and the node formed by the output terminals of the first andsecond high-side switches.
 4. The converter of claim 3, furthercomprising: an inductor bypass switch connected in parallel with theinductor and connected to the control circuit, wherein the inductorbypass switch short-circuits the inductor in response to a controlsignal from the control circuit.
 5. The converter of claim 2, furthercomprising: a feedback path between the control circuit and thenongrounded terminal of the capacitor, wherein the control circuitswitches the selected one of the first and second high-side switches asa function of the voltage at the nongrounded terminal of the capacitor.6. The converter of claim 1, wherein the low-side rectifier is one of adiode and a transistor, and wherein the first and second high-sideswitches are transistors.
 7. The converter of claim 1, wherein thelow-side rectifier has a control terminal, and wherein the controlcircuit causes the low-side rectifier to switch at the same frequency asthe selected high-side switch.
 8. A method of producing a regulatedvoltage from a first supply voltage and a second supply voltage,comprising the steps of: (a) providing the first supply voltage to thefirst high-side switch; (b) providing the second supply voltage to thesecond high-side switch; (c) selecting one of the first and secondhigh-side switches to be a first active switch and the other one of thefirst and second high-side switches to be a first inactive switch; (d)deactivating the high-side switch selected as the first inactive switch;(e) repetitively switching the high-side switch selected as the firstactive switch between a conductive state and a nonconductive state at aswitching frequency to produce a switched output signal, while the firstinactive switch remains in a nonconductive state; and (f) rectifying theswitched output signal to produce a regulated output signal.
 9. Themethod of claim 8, further comprising the step of: (g) deciding totransfer switching from the high-side switch selected as the firstactive switch to the high-side switch selected as the first inactiveswitch; (h) deactivating the high-side switch selected as the firstactive switch; and (i) switching the high-side switch selected as thefirst inactive switch at a switching frequency to produce the switchedoutput signal.
 10. The method of claim 8, wherein step (c) of selectingone of the first and second high-side switches comprises the step ofmonitoring the first supply voltage and the second supply voltage. 11.The method of claim 8, further comprising the step of: (j) feeding backthe regulated output voltage signal to produce a feedback signal; and(k) adjusting the switching of step (e) based on the feedback signal.12. The method of claim 8, wherein the step (f) of rectifying comprisesthe steps of: (g) providing the switched output signal to a low-siderectifier; and (h) switching the low-side rectifier at the samefrequency as the selected high-side switch.